tests.doublefree: Add 'double free' test.

Double free performs a build of doublefree.c and execution,
that triggers a double free() function calling, for the same malloc()
region.

The motivation behind this test is to catch libc errors and log inside
Avocado execution (see bug #79).

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #85 from ruda/doublefree_test

tests.doublefree: Add 'double free' test.
tests: Add trinity wrapper

Trinity is a popular linux system call wrapper

http://codemonkey.org.uk/projects/trinity/

By implementing it we noticed a number of shortcomings
with avocado that have to be fixed in order to provide
a smoother developer experience.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>